Perceptual audio codecs make use of filter banks and MDCT (modified discrete cosine transform, a forward transform) in order to achieve a compact representation of the audio signal, i.e. a redundancy reduction, and to be able to reduce irrelevancy from the original audio signal. During quasi-stationary parts of the audio signal a high frequency or spectral resolution of the filter bank is advantageous in order to achieve a high coding gain, but this high frequency resolution is coupled to a coarse temporal resolution that becomes a problem during transient signal parts. A well-know consequence are audible pre-echo effects.
B. Edler, “Codierung von Audiosignalen mit ütberlappender Transformation und adaptiven Fensterfunktionen”, Frequenz, Vol. 43, No. 9, p. 252-256, September 1989, discloses adaptive window switching in the time domain and/or transform length switching, which is a switching between two resolutions by alternatively using two window functions with different length.
U.S. Pat. No. 6,029,126 describes a long transform, whereby the temporal resolution is increased by combining spectral bands using a matrix multiplication. Switching between different fixed resolutions is carried out in order to avoid window switching in the time domain. This can be used to create non-uniform filter-banks having two different resolutions.
WO-A-03/019532 discloses sub-band merging in cosine modulated filter-banks, which is a very complex way of filter design suited for poly-phase filter bank construction.